Chargeable systems, such as electric vehicles, have been introduced that include locomotion power derived from electricity received from an energy storage device such as a battery. For example, hybrid electric vehicles include on-board chargers that use power from vehicle braking and traditional motors to charge the vehicles. Vehicles that are solely electric generally receive the electricity for charging the batteries from other sources. Battery electric vehicles are often proposed to be charged through some type of wired alternating current (AC) such as household or commercial AC supply sources. The wired charging connections require cables or other similar connectors that are physically connected to a power supply. Cables and similar connectors may sometimes be inconvenient or cumbersome and have other drawbacks. It is desirable to provide wireless charging systems that are capable of transferring power in free space (e.g., via a wireless field) to be used to charge the electric vehicle to overcome some of the deficiencies of wired charging solutions. Additionally, wireless charging system should be capable of determining a position of the electric vehicle and/or the charging system to facilitate the transfer of wireless power to a receiver in a most efficient manner possible.
Inductive power transfer (IPT) systems provide one example of wireless transfer of energy. In IPT systems, a primary device (i.e., the transmitter) transmits power to a secondary device (i.e., the receiver). Each of these transmitter and receiver devices may comprise at least one inductive coupler (e.g., IPT coupler), which may comprise a single coil or a multi-coil arrangement of windings of electric current conveying material. In IPT systems, an alternating current in the transmitter produces a magnetic field, which induces an electromotive force in a receiver placed in proximity to the transmitter and accordingly transfers power to the receiver. Typically, frequencies in the VLF or LF frequency bands (e.g., from 20 kHz to 150 kHz) are used for inductive power transfer for electric vehicle charging.
Inductive charging of electric vehicles in the kilowatt range requires relatively tight coupling to be efficient and compliant with regulatory standards. With inductive charging, higher power comes with tighter coupling. Inductive charging of electric vehicles may be used with static charging systems (where the inductive charging takes place while the electric vehicle is motionless) or dynamic charging systems (where the inductive charging takes place while the electric vehicle is in motion). In static system, park assist systems may help drivers overcome alignment issues and increase convenience and charging efficiencies. For example, the park assist system may indicate to the driver when the electric vehicle is parked within the “sweet spot.” According to another embodiment, the park assist systems may direct the electric vehicle to automatically park itself with minimal driver intervention. This is particularly valuable for position-critical vehicle charging systems. Guidance and alignment systems may be used to assist drivers to reliably park the electric vehicle within a “sweet-spot” or “tolerance area,” which may be an area where the coupling efficiency between the transmitter (on the charging base) and receiver (on the electric vehicle) is at or above a certain threshold or minimum value. The sweet-spot and the charging areas may also be defined according to the emissions that are released into the parking area, e.g. if vehicle is positioned within the sweet spot or tolerance area, the magnetic leakage field as measured in the surrounding of the vehicle satisfies specified limits, e.g., regulatory human exposure limits. Accordingly, systems and methods for providing local positioning based on sensing a low frequency magnetic field that may be generated either by the base charging unit or the electric vehicle charging unit at a frequency preferably below 150 kHz to provide guidance and alignment information are desired.